In a three-point seat belt known from WO 02/06092, a Hall effect sensor is arranged between two permanent magnets. The Hall effect sensor in its starting position is located in the immediate proximity of one the two magnets. The two magnets are mounted on a carrier that is movable in a housing. The Hall effect sensor is fastened to a base plate securely connected to the vehicle structure. The movable magnet carrier is connected to a seat belt. If a child's seat is secured to a vehicle seat by the seat belt, a movement of the two permanent magnets in relation to the stationary Hall effect sensor takes place on the basis of the pulling force acting upon the belt webbing, whereby the Hall effect sensor is brought into the immediate proximity of the other permanent magnet. In this way, the occupation of the seat by a child's seat is indicated and the possibility of inflation of an airbag allocated to this vehicle seat is stopped. The movement of the magnet carrier is carried out against a bias force generated by springs.
WO 01/79039 teaches a measuring device located between an anchoring point positioned on a vehicle and a belt buckle that can be connected to the seat belt and is movable against a spring bias in relation to the anchoring point. The measuring device comprises a magnetic field generated by a permanent magnet, and in which a magnetic field sensor is arranged. Depending on a pulling force acting upon the belt webbing, a movement of the permanent magnet takes place in relation to the magnetic field sensor, the measuring signal of which is electronically evaluated for measuring the pulling force.
U.S. Pat. No. 6,081,759 teaches a force measuring device that is located in the region of the end fitting of a seat belt and measures the pulling force acting upon the belt webbing with a magnetic field sensitive sensor.
U.S. Pat. No. 6,081,759 teaches a measuring device arranged in the region of the end fitting. In a preferred embodiment, the measuring device is located in or on the belt buckle of the seat belt. In this case the magnetic field sensor is securely connected to the belt buckle frame made out of a solid material, for instance metal. A bolt is mounted on the belt buckle frame and during the strapping of the seat belt engages a buckle tongue attached to the belt webbing. The two permanent magnets are secured to a magnet holder, which is securely connected to the anchoring point on the vehicle. Buckle plates of the belt buckle frame can be extended accordingly, to provide the measuring device in between them.